What do you mean "can't assign user and password for yourself"? You can enter any user name and password in User Manager and FreeRADIUS. The user can even create his/her own account in User Manager. FreeRADIUS requires a php script to interface with the radius MySQL database to allow customers to enter their own account.
If you are set on trying the text file parsing, here is part of a script I wrote for GPS that parses file data. You will need to figure out how to parse the user/password from each line. The "\n" in the find statements indicates a new line.
:global gpstext [/file get gps.txt contents];
:local longstart [:find $gpstext "longitude" -1];
:local longend [:find $gpstext "\n" $longstart];
:local latstart [:find $gpstext "latitude" -1];
:local latend [:find $gpstext "\n" $latstart];
:local validstart [:find $gpstext "valid" -1];
:local validend [:find $gpstext "\n" $validstart];